Conventional synthesis gas generating processes include steam reforming, gas phase partial oxidation and autothermal reforming. Each of these processes has advantages and disadvantages when compared to each other.
In a steam reforming process, steam is reacted with a hydrocarbon containing feed to produce a hydrogen-rich synthesis gas. The general stoichiometry, as illustrated for methane, is:CH4+H2O→CO+3H2  (1)Typically, an excess of steam is used to drive the equilibrium to the right. As applied to hydrogen manufacture, excess steam also serves to increase the water gas shift reaction:CO+H2O→CO2+H2  (2)
Because of the high endothermicity of the reaction, steam reforming is typically carried out in large furnaces, in which a reforming catalyst is packed into tubes. The tubes must withstand the high pressure of the produced synthesis gas, while transmitting heat at temperatures approaching 1000° C. As described in Stanford Research Institute International Report No. 212 (1994), steam reforming process efficiency, (defined as the heat of combustion of product hydrogen divided by the heat of combustion of reforming feed and furnace fuel), is approximately 74%, while the space velocity, (defined as Standard Cubic Feet per Hour of C1-equivalent feed/ft3 of catalyst bed) is 1000 hr−1. Unfortunately, steam reforming furnaces occupy a very large volume of space, substantially greater than the tube volume. This feature, and the relatively low efficiency, combine to severely limit its utility in point-of-use fuel applications such as fuel cells and would likely be unfeasible for on-board vehicle applications.
Gas phase partial oxidation involves the partial oxidation of the hydrocarbon containing feed in the gas phase. The feed components are introduced at a burner where they combust with sub-stoichiometric oxygen to produce a synthesis gas mixture. The ideal gas phase partial oxidation reaction, as illustrated for methane, is:CH4+½O2→CO+2H2  (3)However, gas-phase reaction kinetics tend to over-oxidize some of the feed, resulting in excessive heat generation and substantial yield of H2O, CO2, and unreacted hydrocarbons as soot.
For these reasons, when gas phase partial oxidation chemistry is applied to clean feeds, it is preferred to add steam to the feed and add a bed of steam reforming catalyst to the gas phase partial oxidation reactor vessel. This combination of gas phase partial oxidation and steam reforming is called autothermal reforming. A fully catalytic version of autothermal reforming, typically using platinum or rhodium to catalyze the oxidation, is known in the art. However, autothermal reforming requires a source of oxygen. In the fuel cell vehicle applications, this oxygen is typically provided as compressed air, which results in a nitrogen-diluted synthesis gas that adversely effects the operating efficiency of the fuel supply/fuel cell and adds the cost and complexity of an additional compressor.
Sederquist (U.S. Pat. Nos. 4,200,682, 4,240,805, 4,293,315, 4,642,272 and 4,816,353) teaches a steam reforming process in which the heat of reforming is provided within the bed by cycling between combustion and reforming stages of a cycle. As described by Sederquist, the high quality of heat recovery within the reforming bed results in a theoretical efficiency of about 97%. However, these patents describe a process that operates at very low productivity, with space velocities of around 100 hr−1 (as C1-equivalent). Moreover, this process requires a compressor to compress the product synthesis gas to elevated pressure. One consequence of Sederquist's low space velocity is that resulting high heat losses impede the ability of this technology to achieve the theoretical high efficiency.
The inventors here have discovered a highly efficient and highly productive process for producing hydrogen from a hydrocarbon containing fuel. This process is a cyclic, two step process referred to herein as “pressure swing reforming” or “PSR”. The reforming step involves introducing a hydrocarbon-containing feed, along with steam. The feed may also include CO2, and, optionally, other process gases. The feed is introduced to the inlet of the first zone containing reforming catalyst. During the reforming step a temperature gradient across the reforming catalyst has a peak temperature that ranges from about 700° C. to 2000° C. Upon introduction of the reactants, the hydrocarbon is reformed into synthesis gas over a catalyst in this first zone. This reforming step of the cycle may be performed at a relatively high pressure. The synthesis gas is then passed from the first zone to a second zone, where the gas is cooled to a temperature close to the inlet temperature of the regeneration-step feed by transferring its heat to packing material in the recuperation zone.
The regeneration step begins when a gas is introduced to the inlet of the second zone. This gas is heated by the stored heat of the packing material of the recuperation zone. Additionally, an oxygen-containing gas and fuel are combusted near the interface of the two zones, producing a hot flue gas that travels across the first zone, thus re-heating that zone to a temperature high enough to reform the feed. This second part of the cycle is performed at a relatively low pressure. Once heat regeneration is completed, the cycle is completed and reforming begins again.
The PSR process produces a relatively high pressure, hydrogen-containing synthesis gas that may be used to fuel a fuel cell. The PSR process may be integrated with synthesis gas adjustment processes where the fuel cell fuel purity requirements dictate. In one embodiment the PSR process is integrated with a water gas shift reaction and a preferential oxidation (“PROX”) reaction to convert CO to CO2. In an alternate embodiment, a membrane separation means is substituted for, or supplements, the CO conversion reactions. The membrane functions to separate hydrogen from other synthesis gas components (i.e. CO, CO2 and any residual hydrocarbon containing gases). In an alternate embodiment, a pressure swing adsorption step is substituted for the membrane separation step to remove the other synthesis gas components from hydrogen.
The present invention is advantageous in efficiency and in producing relatively high partial pressures of hydrogen fuel when compared to air-blown auto thermal reforming. The present invention is advantageous in efficiency, compactness, hydrocarbon conversion, and reactor cost when compared with other steam reforming approaches. When used in a fuel cell application, the high spatial velocities are advantageous to the efficiency of the fuel supply/fuel cells system.